From 6f6a983a938b0d37e3fe00a723043c56026116fc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andre=CC=81s=20Cecilia=20Luque?=
 <andres.luque@cellpointmobile.com>
Date: Thu, 26 Dec 2019 02:18:27 +0100
Subject: [PATCH] Added the option to throw when a variable used in the
 template is not found in the context

---
 CHANGELOG.md                             |  3 +++
 Sources/Environment.swift                |  3 +++
 Sources/Variable.swift                   |  8 +++++++-
 Tests/StencilTests/EnvironmentSpec.swift | 18 +++++++++++++++++-
 4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 752d7c95..d3c0dc7f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,9 @@ _None_
 , i.e. `myfilter = "uppercase"` and then use it to invoke this filter with `{{ string|filter:myfilter }}`.  
   [Ilya Puchka](https://github.com/ilyapuchka)
   [#203](https://github.com/stencilproject/Stencil/pull/203)
+- Added option to throw when a variable used in a template could not be resolved.  
+  [Andres Cecilia Luque](https://github.com/acecilia)
+  [#289](https://github.com/stencilproject/Stencil/pull/289)
 
 ### Deprecations
 
diff --git a/Sources/Environment.swift b/Sources/Environment.swift
index 0c2c72e7..ba636ce2 100644
--- a/Sources/Environment.swift
+++ b/Sources/Environment.swift
@@ -1,16 +1,19 @@
 public struct Environment {
   public let templateClass: Template.Type
   public var extensions: [Extension]
+  public let throwOnUnresolvedVariable: Bool
 
   public var loader: Loader?
 
   public init(loader: Loader? = nil,
               extensions: [Extension] = [],
+              throwOnUnresolvedVariable: Bool = false,
               templateClass: Template.Type = Template.self) {
 
     self.templateClass = templateClass
     self.loader = loader
     self.extensions = extensions + [DefaultExtension()]
+    self.throwOnUnresolvedVariable = throwOnUnresolvedVariable
   }
 
   public func loadTemplate(name: String) throws -> Template {
diff --git a/Sources/Variable.swift b/Sources/Variable.swift
index 44569dff..39f3fff8 100644
--- a/Sources/Variable.swift
+++ b/Sources/Variable.swift
@@ -30,10 +30,16 @@ class FilterExpression: Resolvable {
   func resolve(_ context: Context) throws -> Any? {
     let result = try variable.resolve(context)
 
-    return try filters.reduce(result) { value, filter in
+    let filteredResult = try filters.reduce(result) { value, filter in
       let arguments = try filter.1.map { try $0.resolve(context) }
       return try filter.0.invoke(value: value, arguments: arguments, context: context)
     }
+
+    if filteredResult == nil, context.environment.throwOnUnresolvedVariable {
+      throw TemplateSyntaxError("Variable '\(variable.variable)' could not be resolved")
+    }
+
+    return filteredResult
   }
 }
 
diff --git a/Tests/StencilTests/EnvironmentSpec.swift b/Tests/StencilTests/EnvironmentSpec.swift
index f5b829ff..06bc2a35 100644
--- a/Tests/StencilTests/EnvironmentSpec.swift
+++ b/Tests/StencilTests/EnvironmentSpec.swift
@@ -182,7 +182,23 @@ final class EnvironmentTests: XCTestCase {
     }
   }
 
+  func testUnresolvedVariable() {
+    self.template = Template(templateString: "Hello {{ name }}")
+
+    it("can render a variable not defined in the context and throwing is disabled") {
+      self.environment = Environment()
+      let result = try self.environment.render(template: self.template, context: [:])
+      try expect(result) == "Hello "
+    }
+
+    it("throws when a variable is not defined in the context and throwing is enabled") {
+      self.environment = Environment(throwOnUnresolvedVariable: true)
+      try self.expectError(context: [:], reason: "Variable 'name' could not be resolved", token: "name")
+    }
+  }
+
   private func expectError(
+    context: [String: Any] = ["names": ["Bob", "Alice"], "name": "Bob"],
     reason: String,
     token: String,
     file: String = #file,
@@ -192,7 +208,7 @@ final class EnvironmentTests: XCTestCase {
     let expectedError = expectedSyntaxError(token: token, template: template, description: reason)
 
     let error = try expect(
-      self.environment.render(template: self.template, context: ["names": ["Bob", "Alice"], "name": "Bob"]),
+      self.environment.render(template: self.template, context: context),
       file: file,
       line: line,
       function: function